A screen printing method has been used for printing paste such as cream solder onto a substrate in a parts-mounting-process by a part-mounting-machine. This method works as this: first, overlay a maskplate on a substrate, the mask-plate has pattern holes corresponding to spots to be printed on the substrate. Then supply cream solder on the mask-plate, and slide a squeegee. Cream solder is thus printed onto the target spots through the pattern holes.
A highly accurate printing by this printing method would require the following two points:
First, a substrate must be accurately positioned with respect to a mask plate. In other words, the substrate must be accurately positioned to the mask plate in a horizontal direction, and the substrate must solidly adhere to the lower face of the mask plate. Therefore, when the apparatus starts on or a model is changed in the mounting line, various adjustments must be done in order to position a substrate accurately to the mask plate. The adjustments include preparing machine-parameters of a moving table of a substrate-positioning-section.
Second, the cream solder must be filled up in the pattern holes when the squeegee is slid, and various printing conditions should be set appropriately to an object to be printed. For instance, a squeegeeing speed on the screen mask, a pressure urging the squeegee against the screen mask and other parameters are to be prepared depending on the characteristics of the object to be printed.
These printing conditions have been set by skilled workers based on their experience and personal know-how. In other words, before actual printing starts, fill-in condition of the cream solder is inspected with human eyes on a trial printing stage, then the condition is corrected by the skilled worker based on the experience and intuition. The printing conditions are thus manually prepared.
However, even if these conditions are adjusted correctly, the substrates are not always positioned exactly with the mask plate at actual printing. There are dispersions on dimensions of the substrates, aged deformation of the mask plate, and looseness of mounting the mask plate. These factors would deviate the pattern holes from the correct positions. Further, gaps would be produced between the substrate and the lower face of the mask plate.
In these cases, defects such as “deviation” of a printed spot from the target one or “blur” of the paste from the printed spot are produced. As such, in the conventional printing method, deviation of the substrate from the mask plate tends to occur, and it is difficult to keep a stable printing quality.
In screen printing, since the squeegee is horizontally moved while it is urged to the mask plate, an external force is applied to the mask plate so that the mask plate is pulled by the squeegee in the lateral direction. This external force repeatedly works on the mask plate at every printing operation, which could loosen the mounting condition of the mask plate or extend the mask plate. As a result, even if the substrate is correctly positioned, the movement of the squeegee at every printing deviates the mask plate relatively from the substrate, and a printing position cannot be exactly maintained.
In these days it becomes difficult to keep the skilled workers. Further, limited production of a variety of products becomes more popular, thus the printing conditions must be changed frequently at every model change. Since the printing conditions are manually prepared, the frequent changes take time and labor, which, as a result, prevents the productivity from improving.
Once the printing conditions are set, the fill-in condition of cream solder is not necessarily kept stable because the cream solder changes over the time, so that the printing quality becomes unstable.
As such, the conventional screen printing apparatus has problems such as (1) preparing the printing conditions takes time and labor, (2) it is difficult to stabilize the printing quality due to dispersion of the printing conditions.